Embodiments of the invention relate to a technique for reducing artifacts appearing in an image produced by a radiation tomography apparatus.
Aliasing artifacts are known as one kind of artifact appearing in images produced by radiation tomography apparatuses. The aliasing artifacts are affected by the spatial resolution of a radiation detector, focus size of a radiation source, number of data samples, and the like, in a radiation tomography apparatus, and they are generated mainly in case that a subtle difference occurs in reproduction of the shape of edges or a minute shape in an object, which is a target to be imaged. More particularly, this may be explained as follows:
Let us consider a radiation detector having detector elements arranged with fine pitch and providing very good spatial resolution. In this case, the shape of an object obtained at the radiation detector is consistent with little discrepancy when imaged in any view direction. On the other hand, consider a radiation detector having detector elements arranged with coarse pitch and providing poor spatial resolution. In this case, the shape of an object obtained at the radiation detector has a varying degree of inconsistency depending upon the positional relationship between the object and radiation detector. Specifically, the shape of an object may inconsistently look somewhat broad in a certain view direction or sharp in another view direction, resulting in a difference from view direction to view direction. Such a difference may often occur in case that a high-contrast border of an object significantly changes due to a small difference in view angle. The inconsistent information therefrom causes streak artifacts on the borderline of the object. This is the origin of aliasing artifacts. The artifacts thus resulting from inconsistent information is usually variable, and therefore, it is difficult to cancel out the artifacts by a filtered back-projection technique.
The phenomenon of generation of aliasing artifacts is a fundamental one based on a mutual relationship between a structure related to the spatial resolution of a radiation detector and conditions in sampling by the detector, and the like, which cannot be avoided.
Conventional measures against the phenomenon include, for example, a methods involving adjusting spatial frequency characteristics of a reconstruction filter (also referred to as reconstruction function) for use in image reconstruction processing to make the aliasing artifacts inconspicuous.